1. Field
The present invention relates to energy conversion and specifically to circuitry which combines multiple voltage inputs from serially connected direct current sources into a combined output.
2. Description of Related Art
Sunlight includes a spectrum of electromagnetic radiation emitted by the Sun onto the surface of the Earth. On the Earth, sunlight is filtered through the atmosphere, and the solar irradiance (Watts/meter square/nanometer W/m2/nm) is obvious as daylight when the Sun is above the horizon. The Earth receives a total solar irradiance determined by its cross section (π·RE2, RE=radius of the earth), but as the Earth rotates the solar energy is distributed across the entire surface area (4·π·RE2). The solar constant is the amount of incoming solar electromagnetic irradiance per unit area, measured on the outer surface of Earth's atmosphere in a plane perpendicular to the solar rays. The solar constant is measured by satellite to be roughly 1366 watts per square meter (W/m2) or 1.366 W/m2/nm. Hence the average incoming solar irradiance, taking into account the angle at which the rays strike and that at any one moment half the planet does not receive any solar irradiance, is one-fourth the solar constant (approximately 0.342 W/m2/nm). At any given moment, the amount of solar irradiance received at a location on the Earth's surface depends on the state of the atmosphere and the location's latitude.
The performance of a photovoltaic cell depends on the state of the atmosphere, the latitude and the orientation of the photovoltaic cell towards the Sun and on the electrical characteristics of the photovoltaic cell.
FIG. 1 shows schematically a graph of a solar irradiance 100 versus wavelength. Irradiance 100 is distributed around a peak wavelength at about 550 nanometers. FIG. 1 also shows schematically an absorption spectrum 102 of a typical solar photovoltaic (PV) cell with a given band-gap which allows only a portion of the solar irradiance to be converted into electrical power. The finite characteristic of the band-gap of the photovoltaic cell causes a substantial part of the sun's energy to remain unutilized. In order to improve photovoltaic efficiency, multiple junction cells have been designed which include multiple pn junctions. Solar irradiance not absorbed, because its energy is less than the band gap is transmitted to the next junction(s) with a smaller band gap and the transmitted radiation is preferentially absorbed and converted into electrical energy.
FIG. 2 shows the graph of solar irradiance 100 versus wavelength and three absorption spectra 202, 204 and 206 respectively of three photovoltaic junctions used in a single multi-junction cell designed to absorb different parts of the solar spectrum. The first photovoltaic junction having the largest band gap has an absorption spectrum 206, the second photovoltaic junction has an absorption spectrum 204, and the third photovoltaic junction which has the smallest band gap has an absorption spectrum 202. Combining the three pn junctions of photovoltaic junctions into a single multi-junction 30 cell increases the efficiency, theoretically to about 60% and practically today to above 40%.
FIG. 3 illustrates multiple multi-junction cells 30 connected in series. Each multi-junction cell 30 has three serially connected photovoltaic junctions 300, 302, and 304 which operate with three absorption spectra 206, 204 and 202 respectively. Multiple multi-junction cells 30 connected in series form a multi-spectral photovoltaic panel 3000 with output terminals 310 and 308.
FIG. 4 illustrates characteristic current-voltage curves of a single photovoltaic junction cell at different illumination levels. Curve 400 shows the maximum power point (MPP) for low light levels, curve 402 show the maximum power point MPP for higher light levels, and curve 404 shows the maximum power point MPP yet higher light levels assuming a constant temperature of the cell. As can be seen, at the different light levels the maximum power point is achieved at nearly identical voltages, but at different currents depending on the incident solar irradiance.
Reference is now made to conventional art in FIG. 5a and 5b which shows a typical photovoltaic installation 50 operating in dark or partially shaded conditions and bright mode respectively. Bypass diodes 500a-500c are connected in parallel across photovoltaic panels 502a-502c respectively for instance according to IEC61730-2 solar safety standards (sec. 10.18). Photovoltaic panels 502a-502c are connected in series to form a serial string of photovoltaic panels. Referring to FIG. 5a, bypass diode 500a provides a path 510 around photovoltaic panel 502a during dark or partially shaded conditions. Current path 510 allows current to flow through bypass diode 500a in the forward mode, preventing common thermal failures in photovoltaic panel 502a like cell breakdown or hot spots. During forward mode, bypass diode 500a preferably has low forward resistance to reduce the wasted power. FIG. 5b refers to normal operation or bright mode, forward current 512 will flow through photovoltaic panels 502a-502c while bypass diodes 500a-500c will operate in the reverse blocking mode. In reverse blocking mode, it is important that bypass diodes 500a-500c have the lowest high temperature reverse leakage current (IR) to achieve the highest power generation efficiency for each photovoltaic panel 502a-502c. 